On-line and in-line analytical measurements are routinely performed for environmental and industrial process monitoring and control. Many of the specific measurements made in these fields are performed using spectroscopic-probes, which are inserted into the sample to be analyzed. These types of probes are generally referred to as xe2x80x98immersion probes.xe2x80x99 These probes are found in many shapes, sizes and optical configurations specific to a certain process or sample being analyzed. The need for the multitude of different optical probe designs stems from the varying samples they are designed to measure. These samples range from pure liquids, pastes, slurries, powders, solids and gases at varying temperatures, pressures and pH""s.
Many immersion probe designs are intricately engineered with numerous moving parts and optical components. The addition of moving parts to allow an operator to align/focus a probe leads to imprecision during repeated analyses. The measurement errors may be due to misalignment, focus/alignment drifting over time or numerous operators having different optimization criteria. The addition of multiple optical interfaces can also lead to measurement imprecision when using immersion probes. Flat faced optical probes have a tendency to collect material on the optic in contact with the sample, thereby degrading performance over time. Many probes use a focusing (movable) optic in the barrel of the probe that is focused through a flat window that is in contact with the sample. Most immersion probes operate with a focusing lens that illuminates a portion of the sample that is some fixed distance from the physical tip of the probe (either window or lens). This common probe design leads to imprecision due to changing focal length and tip fouling and increases light scattering due to particles in a sample, changes in optical density and other physical variations in sample properties.
There is a need in the art for a single robust, straightforward, versatile and precise optical probe for use with various spectroscopic techniques to analyze all types of samples. The fact that the focal volume of the probe is a constant at the surface of the optical element in contact with the sample ensures accurate optical focus with whatever type of sample is present.
This invention provides a number of attributes not available in known optical immersion probes: 1) precise focus on any surface or material; 2) no need for sample alignment; 3) ease of samplingxe2x80x94simply place probe into or onto sample; 4) ability to be used in flowing/static sampling systems; 5) analysis not affected by directional flows or variable contact points; 6) analysis not affected by differential light scattering or particle distribution of solid particles; and 7) fully sealed probe element that is highly durable in harsh process/analytical environments. Thus, this invention circumvents the need for a multitude of imprecise complicated optical probes to measure samples ranging from gases to liquids to solids.
A novel optical probe is provided utilizing a spherical lens as both the optical and sample interface for applications including laboratory and process analysis applications. The spherical lens optical immersion probe (also called a ball probe) is an efficient sampling interface for the analysis of many types of samples including solids, powders, slurries, suspensions, particles, vapors, liquids and the like. The samples may be homogeneous, heterogeneous, or comprised of multiple phases. The probe design is compact, durable and straightforward with no moving or easily fouled components. The spherical lens probe has been demonstrated to greatly improve the precision of spectroscopic measurements (e.g. ultraviolet/visible (UV-Vis), near-infrared (NIR), mid-infrared (FTIR), fluorescence, and Raman) of a variety of samples over other known optical immersion probes. Importantly, this invention has broad applications to any optical analytical technology that necessitates an optical immersion probe.
The precision of the optical immersion probe of this invention is due to its novel design whereby a spherical lens is used as both the light focusing element and the optical interface with the sample. As such, the probe may also serve as a light collecting lens or device for optical signal collection. For example, in Raman spectroscopy, scattered light is collected by the spherical lens and directed to the instrumentation for analysis. This optical design provides a constant and precisely positioned focal volume, located directly on the proximal face of the spherical optic, for the excitation source of the various optical analyses, which leads to greatly increased measurement precision.
Further precision enhancement is gained by choosing a spherical lens having a focal point close to the surface of the spherical lens. Typically, the focal point is from about 50 xcexcm to about 200 xcexcm from the apex of the spherical lens. This ensures that any sample in contact with the spherical lens is properly focused to perform an optimal optical measurement. This design element eliminates the measurement imprecision due to path length variations inherent in other optical immersion probe designs.
FIG. 1 shows the theoretical optical path of a collimated optical beam through the spherical lens. For example, a focal length of about 200 xcexcm from the apex of the spherical lens surface has been determined when a collimated 3 mm diameter 785 nm laser beam illuminates the surface of the spherical lens opposite the surface in contact with the sample. There is also no need for optical focusing of this probe onto/into the sample because the sample is optimally focused when it is in contact with the spherical lens. This makes the spherical lens optical immersion probe of this invention a focus free immersion probe with the only sampling condition being that the spherical lens itself must be in contact with the sample.
In its simplest embodiment, the immersion probe of this invention comprises a spherical lens attached to one end of a cylinder (the probe tip) in such a way that the end of the cylinder immersed in the sample (herein the xe2x80x98proximal endxe2x80x99) is substantially sealed and leak-proof. The seal can be provided by welding or braising the lens to the probe tip, or by using epoxy or other adhesives to fix the lens to the cylinder. Preferably the seal is provided by braising a sapphire lens to a metal or alloy cylinder. In other preferred embodiments, the lens is secured at the proximal end of the probe tip by using a combination of gaskets or o-rings and additional threaded tubes to provide force to the gaskets sufficient for a leak-proof seal.
As used herein, the term xe2x80x98gasketxe2x80x99 is used to refer to a pressure/tight seal made of any deformable material such as polymers, rubber, plastic, metals such as copper and gold, etc. Gaskets can be any shape, including the specific round shape of an o-ring.
Throughout the specification the term xe2x80x9cleak proof sealxe2x80x9d or substantially leak proof is used to describe a seal sufficient to close the interfaces in the optical immersion probe so as to prevent material from entering (or leaving) the interior of the optical immersion probe. The seal must be sufficient to prevent corruption of the analytical results. The quality of the seal is a measure of how much pressure the seal can withstand without leaking and is dictated in part by choice of sealing material (epoxy, weld, o-ring composition, etc. Those skilled in the art are readily able to recognize how to choose and apply materials that will provide sufficient seals for a given application. For example, the immersion probe described in this disclosure was constructed using 316 stainless steel tubing, Chemraz(copyright) 505 o-rings (Green Tweed, Inc.), and a synthetic sapphire spherical lens. The use of Chemraz(copyright) 505 o-rings has been shown to provide an optical immersion probe that is leak-proof to greater than 600 psi Helium. Alternative embodiments using sapphire braising resulted in an immersion probe that is leak proof to greater than 1000 psi Helium.
Embodiments described below use tubing that is circular in cross-section, but tubing having any geometric shape in cross-section may be adapted for use in the invention.
In preferred embodiments, tubes are comprised of metals or metal alloys. Preferred materials include stainless steel and Hastelloy(copyright). However, any material may be used, including plastics, ceramics, ceramic composites, glass, or other materials known in the art. Tubes may be either rigid or flexible. In probes where the excitation source is provided as a collimated beam, the tube material should be sufficiently rigid so that optical alignment of the excitation source with the lens is maintained. Flexible tube materials may be used when the excitation source is directly coupled to the spherical lens, as in the case of a fiber optic cable positioned within the tube, for example.
Gaskets or o-rings are typically rubber, but may also be any elastomeric material or metal capable of conforming to the spherical lens so as to provide a sufficient seal for the application. Factors to consider in the choice of o-rings include chemical compatibility, compressibility, temperature resistance, structural strength, etc. as dictated by the application environment and as known to one of ordinary skill in the art.
Preferably, tubing and gasket materials are selected from materials that are substantially chemically resistant to the chemical environment in which the probe is used. That is, the materials resist corrosion that could lead to failure of the structure (such as the seal) or interference with the measurement. Such materials are known in the art, and one skilled in the art would be able to select materials appropriate for different chemical environments. Tubing and gaskets are also chosen to withstand temperatures and pressures encountered during the analysis.
In preferred embodiments, a sapphire spherical lens was used because of sapphire""s resistance to scratching and transparency over a wide range of wavelengths. The sapphire lens may be synthetic. However, any lens material known in the art may be used including, but not limited to fused silica, glass, doped glass, ruby, diamond, cubic zirconia, zinc selenide, potassium bromide crystal and sodium chloride crystal. Impurities and/or crystalline defects may or may not be present in the lens materials. Similarly, the size of the spherical lens can be readily chosen by one skilled in the art when considering the wavelength of light, optical geometric compatibility and the desired focal length/volume. Furthermore, the term xe2x80x98sphericalxe2x80x99 as used herein refers to any rounded object approximating the form of a geometric sphere as long as the optical properties of that object are sufficient to carry out the invention as described.
The immersion probe utilizing a sapphire spherical lens has been demonstrated for the analysis of various solids, powders, slurries, suspensions, particles, vapors and liquids with very good analytical performance.